I'm planning an OC Hogwarts student costume for the release of the final movie this summer. It's not that I don't like the canon characters, but I don't look enough like any of them to really pull it off. Rather than be one of eleven hundred Hermiones, I'm going to be one of only a couple of Hufflepuff students (no one loves Hufflepuff ;3. But there again... it works with that fandom. There are likely hundreds of unnamed students/extras. The same is true of something like Star Wars. That said, I wouldn't try to cosplay an OC Sailor Scout, or something that people will likely get bent out of shape about.